Field of the Invention
This invention relates to methods for compensating for deterioration of turbine blade tip clearances in a gas turbine engines.
Engine performance parameters such as thrust, specific fuel consumption (SFC), and exhaust gas temperature (EGT) margin are strongly dependent upon clearances between turbine blade tips and static seals or shrouds surrounding the blade tips. Over the duration of the engine's life, these clearances tend to increase as a result of blade rubs, oxidation, and erosion, thus, causing a deterioration in engine performance. It is highly desirable to compensate for this deterioration.
It is well known that a major factor in deterioration of engine efficiency and increasing specific fuel consumption of an aircraft gas turbine engines is the gradual increase in the clearance between the turbine blade tips and surrounding static seals or shrouds. Deterioration of the tip clearances increases the amount of leakage of working fluid of the turbine across individual rotor stages in the turbine as well as compressor stages of the gas turbine engine. Such leakage reduces overall engine efficiency hence raising the total specific fuel consumption.
This increase is directly related to the accumulated usage of an engine since the engine was first placed into service or after engine maintenance was performed to return the tip clearances at or near to original specifications. This typically involves replacement or refurbishment of the seal and/or blade tips, a time consuming and expensive procedure. The blade tip clearances are at a minimum and sealing effectiveness is at a maximum immediately following installation. The blade tip clearances and sealing effectiveness deteriorate as the engine is operated over an ever increasing number of cycles.
One method of improving both the wear life and effectiveness of blade tip seals is “active clearance control”. Active clearance control modulates a flow of cool air from the engine fan and/or compressor which is sprayed on a high pressure turbine casing to shrink the casing relative to the high pressure turbine blade tips under certain operating conditions, i.e. steady state, high altitude cruise conditions. The cooling air may be flowed to or sprayed on other static structure used to support the shrouds or seals around the blade tips. The modulated flow of cooling air is used at preselected engine operating conditions and is designed to allow the engine to operate with minimum seal clearance for the majority of its operating cycle while reducing or eliminating the interference or abrasion between the seal and blade tips which can occur during transient conditions such as takeoff, throttle back, etc.
Engines equipped with active clearance control are still subject to accumulated engine usage related blade tip clearance deterioration. It is known to attempt to measure the operating blade tip clearances and modify the clearances by either mechanical or thermal means. It is also known to periodically restoring the blade tip clearance by incrementally increasing the flow of external cooling air responsive only to an accumulated engine use parameter. Such a method is disclosed in U.S. Pat. No. 4,856,272. This method establishes an incremental change in the engine's active clearance control system at a predetermined engine usage interval between scheduled overhauls. The increment and usage interval are predetermined from prior experience with a plurality of similarly configured engines, allowing the adjustment to a particular engine to be made based solely on the accumulated hours or cycles of engine operation.
It is highly desirable to be able to maintain or restore as much as possible optimum blade tip clearance in an aircraft gas turbine engine between seal and/or blade tip replacement or refurbishment. It is also highly desirable to accurately and automatically compensate for the deterioration in engine performance due to increase blade tip clearance due to wear.